چکیده انگلیسی

The purpose of this study was to investigate the aftereffects of self-generated mental imagery of an effortful task on physical self-control endurance and muscle fatigue. Participants performed two isometric handgrip endurance trials (50% of maximum contraction) separated by either an imagery manipulation or a quiet rest period. The imagery group showed greater negative changes in endurance performance from trial 1 to trial 2 (p = .003, d = 0.87) and increased muscle activation at baseline (p = .01, d = 0.73) and at 25% (p = .03, d = 0.61) of the second endurance trial compared to controls. We conclude that imagined performance of an effortful task depletes self-control strength and contributes to muscle fatigue.

مقدمه انگلیسی

The ability to simulate physical, emotional and cognitive sensations using mental imagery is a fascinating phenomenon and powerful tool that allows us to manipulate many different types of experiences. While some of these experiences may be very straightforward, easy to image, and relaxing (e.g., sipping a cocktail on a tropical beach) others can be more complex; requiring investment of working memory and cognitive effort that can leave one feeling drained (e.g., packing up a house on moving day) (Moran, Guillot, MacIntyre, & Collet, 2012). Considering the latter example, even though no physical action may occur, the experience of imagined events may tax our cognitive, emotional, and physical resources. This hindering of energy resources is akin to the depletion of self-control strength described in the strength model of self-regulation (Baumeister, 2002).
Self-regulation or self-control refers to the ability to exert control over one's behaviors, thoughts, or emotions (Muraven & Baumeister, 2000). The strength model of self-control posits that when people engage in acts requiring self-control they deplete a limited central nervous system (CNS) resource that detracts from their ability to utilize self-control resources for subsequent acts. A meta-analysis by Hagger, Wood, Stiff, and Chatzisarantis (2010) has shown that depletion of self-control strength occurs across similar (e.g., cognitive–cognitive, Cohen's d = 0.59) and dissimilar domains (e.g., emotional–physical, d = 0.63).
Many different tasks have been used to deplete self-control strength and assess the aftereffects of strength depletion. One task that has been frequently used to deplete self-control resources is the “white bear” task (Wegner, Schneider, Carter, & White, 1987), which requires participants to initially form the mental image of a white bear and then suppress (inhibit) that image for a period of time (e.g., 5 min). Based on data from 19 studies reviewed in a meta-analysis by Hagger et al. (2010), participants who suppressed the image of a white bear performed worse on subsequent tasks involving physical, cognitive, and emotional self-control with an average effect size of d = 0.65.
Although ample research supports the notion that suppressing images depletes self-control strength, a relevant question is whether the process of forming mental images depletes self-control strength in a similar way. A study by Ackerman, Goldstein, Shapiro and Bargh (2009) supports the notion that forming images depletes self-control strength. In two studies, participants were instructed to read a story and take the perspective of a character in the story that was exerting self-control. Their findings revealed that participants who had imagined exerting self-control “in the character's shoes” subsequently performed worse on tasks requiring self-control compared to participants who simply read the same story. These findings suggest that self-control strength is vulnerable to both real and imagined depletion. However, it is unclear whether controlling images of oneself performing an activity requiring self-control can deplete self-control resources in a similar manner to that which occurs when imagining someone else.
To test this question, Graham and Bray (2012) had participants perform two self-control tasks (endurance handgrip performance) that were separated by a guided imagery session, a listening task, or a quiet rest period. Contrary to expectations, findings showed no differences between conditions in endurance performance following the imagery manipulation. However, those authors noted that different types of imagery (i.e., scripted versus self-generated) and imagined tasks may require varying levels of self-control resources. For instance, in their study participants were exposed to a guided imagery script describing a moderately-intense aerobic workout session. When interpreting their findings, those authors questioned whether imagining oneself engaging in moderately-intense exercise requires self-control and suggested that imaging tasks that involve a greater amount of self-control strength to perform, such as intense, fatiguing, endurance exercise, may cause greater depletion of self-control strength. Thus, investigation of the effects of performing mental imagery on self-control strength depletion through the use of self-generated imagery of a challenging physical endurance task is required.
The investigation of whether mental imagery depletes self-control resources is important for practitioners as it is not uncommon for sport scientists and rehabilitation practitioners to prescribe the use of mental or motor imagery to aid in the learning or performance of tasks that require the exertion of physical effort (e.g., Cumming and Williams, 2012 and Mulder, 2007). Understanding the effects of imagery on self-control strength and performance is also an important undertaking for empirical reasons as the proposed positive effects of imagery on physical task performance are not universal. For example, a meta-analysis showed the effects of imagery on physical endurance tasks are small and inconsistent (Driskell, Copper, & Moran, 1994). In addition, under some conditions, imagery has even produced negative effects on subsequent performances (e.g., Beilock et al., 2001, Beilock and Gonzo, 2008 and Woolfolk et al., 1985). Considering the mixture of conflicting findings, it is not surprising that specific theorizing or exploration of mechanisms explaining why imagery may hinder subsequent physical performances under certain conditions is elusive. Research that focuses on internal biological factors during and after imagery could assist in understanding why these negative performance aftereffects occur.
Although several theories have been proposed to account for the effects of mental imagery on physical performance, two distinct perspectives are evident in the literature: central and peripheral (cf. Mulder, 2007). The central perspective of imagery suggests that engaging in imagery of physical tasks leads to activation of neurons in various structures of the CNS (e.g., primary motor cortex, premotor cortex, basal ganglia, cerebellum, parietal cortex, and the prefrontal cortex) that are responsible for the execution of the movement (Jeannerod, 2001 and Mulder, 2007). In other words, imagery creates a central organization of a motor program and the associated activation of neurons within various areas of the brain responsible for priming the execution of the motor command is what is thought to lead to increased performance and learning through repeated imagery use. According to this perspective, muscle activity (EMG) should not occur during imagery as such activation should be suppressed upstream of the neuromuscular junction and any evidence of muscle activation is attributable to random activation as a consequence of incomplete motor command inhibition (Jeannerod, 1994 and Jeannerod, 2001).
Theories advocating the peripheral perspective stem from the psycho-neuromuscular theory (e.g., Carpenter, 1894 and Jacobson, 1932), which suggests that performing imagery of a particular movement causes central organization of a motor program as well as activation of motor units in the muscles involved in the actual movement execution, but at a lower magnitude than what the movement would involve. Several studies have supported the peripheral perspective, showing an increase in muscle activity (EMG amplitude) during the imagery session (e.g., Bakker et al., 1996 and Jacobson, 1932). However, several studies have failed to show muscle activation during imagery (e.g., Mulder et al., 2005 and Mulder et al., 2004). Thus, support for the peripheral perspective of imagery remains controversial.
The primary objective of the present study was to investigate the effects of self-generated mental imagery of an effortful task on physical self-control endurance. Based on the discussion above and previous findings by Ackerman et al. (2009), it was hypothesized that engaging in an imagery task involving self-controlled effort regulation would lead to diminished performance on a subsequent self-control task. The second objective was to evaluate the peripheral perspective of imagery as well as how its predictions specifically relate to self-control depletion. We pursued this objective by recording muscle activation (EMG amplitude) throughout an experiment involving two test trials of an isometric muscular endurance task separated by a session of mental imagery of oneself performing the same endurance task. Examining muscle activation during the test trials allowed us to assess trial-to-trial changes in amplitude to determine variations in muscle fatigue following imagery or a quiet rest control task. EMG recordings also allowed us to determine if greater muscle activation was present when participants were engaging in imagery vs. quiet rest and could thus account for variations in self-control depletion or fatigue that might occur across trials.

نتیجه گیری انگلیسی

In sum, the present study revealed that performing self-generated imagery of a task involving self-controlled effort regulation led to impairments in physical endurance performance. Furthermore, evidence of muscle fatigue (increase in proportional EMG amplitude) during the second physical endurance task was not attributable to muscle activation that occurred during imagery. Overall, the results account for prior findings showing that the negative performance outcomes following imagery may be attributed to self-control strength depletion and support the limited strength model of self-regulation (Baumeister, 2002). Results may have implications for practitioners whose clients or patients are encouraged to use imagery as part of performance or therapeutic treatment plans. Future research is encouraged to monitor both brain and muscle activity during, and following, imagery in order to gain a more in-depth understanding of the mechanisms involved in the imagery-performance relationship.